#include<stdio.h>
int lt2_3(){
	double sum = 1;
	int add = 3;
	int sign = -1;
	double temp = 0;
	do{
		temp = 1.0 / add*sign;
		sum += temp;
		sign *= -1;
		add += 2;
	} while (temp >= 1e-6);
	printf("%.6f", sum);
	return 0;
}